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Method and apparatus for controlling power drawn from an energy converterMethod and apparatus for controlling power drawn from an energy converter description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070040540, Method and apparatus for controlling power drawn from an energy converter. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS-REFERENCE TO RELATED APPLICATION [0001] This application is a divisional of application Ser. No. 10/672,075, filed Sep. 29, 2003, now U.S. Pat. No. 7,091,707, the disclosure of which is hereby expressly incorporated by reference. FIELD OF THE INVENTION [0002] This invention relates to energy conversion and more particularly to methods and apparatus for controlling power drawn from an energy converter operable to convert energy from a physical source into electrical energy. BACKGROUND [0003] Energy conversion devices such as photovoltaic arrays are commonly used to provide power to electrical loads. Often these loads are direct current (DC) loads such as batteries, for example. Recently, efficiencies in power conversion devices are giving rise to solar power systems that supply power to an alternating current (AC) load such as an AC power grid such as may be operated by a public utility company. Such power systems may employ a photovoltaic array and an interface for converting power in a form received from the photovoltaic array into a form operable to be received by the AC power grid. Such an interface may involve a DC to AC inverter. [0004] Interfaces of the type described above often seek to cause maximum power to be provided to the AC power grid. The maximum power available to be provided to the AC power grid depends upon the conditions under which the energy conversion device is operated and in the case of a photovoltaic array, these conditions include the amount of insolation and the temperature of the array, for example. A maximum power point, or voltage at which maximum power may be extracted from the array, is a desirable point at which to operate the array and conventional systems seek to find this point. The maximum power point changes however, due to changes in insolation and due to changes in temperature of the array and thus control systems are employed to constantly seek this point. [0005] One way of seeking the maximum power point is to periodically perturb and observe the power output of the array and then adjust the power demanded from the array accordingly to cause the voltage of the array to be as close as possible to the maximum power point. Typically, such perturb and observe methodologies involve perturbing the present power supplied to the load by a fixed amount such as 4 watts, for example and then observing the effect on power supplied by the array and the voltage measured at the array. Perturbing involves temporarily increasing the power supplied to the load by a fixed amount such as 4 watts, for example. If the change in power is negative and voltage measured at the array drops by a significant amount, too much power is being extracted from the array and the power demand on the array must be reduced, in which case the power supplied by the array is usually reduced by some fixed incremental value, such as 4 watts, for example. If the voltage does not change by a significant amount when the power is perturbed, perhaps not enough power is being extracted from the array and the present power drawn from the array must be increased in which case the power demanded from the array is usually increased by a fixed amount, such as 4 watts. [0006] The above described perturb and observe methodology is typically conducted at the switching speed of a switching mode power supply connected to the array, e.g., 100 kHz, and results in a dithering of power drawn from the array, in fixed amounts. Where the incremental amount is 4 watts for example, as described above, there will be a constant dithering of power demanded from the array, in the amount of 4 watts about a common mode value which may be approximately equal to the maximum power output of the array. When the load is an AC power grid, the load effectively fluctuates at the line frequency of the grid, which in North America is typically 60 Hz. Consequently, the 100 kHz perturb and observe frequency of most switching mode power supplies used to supply DC loads is too fast for applications where the load is an AC power grid. Thus, the perturb and observe frequency must be decreased. However, decreasing the perturb and observe frequency can waste power, especially when changes in insolation occur. [0007] Changes in insolation can change the maximum power available from the array from say 200 watts to 2000 watts in a matter of seconds. This situation may occur when a cloud, for example, moves or dissipates from a position blocking sunlight shining on the array to a position in which full sun is received on the array. With 4 watt power increments, and a perturb and observe period of 50 mSec, the time to change the power drawn from the array from 200 watts to 2000 watts would be about 22 seconds. During this period the full available power is not being drawn from the array resulting in inefficient operation. SUMMARY [0008] This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. [0009] In accordance with one aspect of the invention there is provided a method of controlling power drawn from an energy converter to supply a load, where the energy converter is operable to convert energy from a physical source into electrical energy. The method involves changing the amount of power drawn from the energy converter when a supply voltage of the energy converter meets a criterion, said criterion and a change in the amount of power drawn from the energy converter being dependent upon a present amount of power supplied to the load. [0010] The method may involve measuring the supply voltage. [0011] Changing the power drawn from the energy converter may include decreasing the power drawn from the energy converter by an amount corresponding to a change in the power supplied to the load in a time interval, or it may include increasing the power drawn from the energy converter by an amount associated with a range of power supplied to the load. [0012] The method may involve deeming the supply voltage satisfies the criterion when the supply voltage is within a first range of voltages relative to a reference voltage. The reference voltage may correspond to a maximum power point of the energy conversion device. The first range may include voltages greater than the reference voltage, or the first range may include voltages less than the reference voltage. Alternatively, the first range may include voltages less than the reference voltage and voltages greater than the reference voltage. The first range may exclude a range of voltages within a limit of the reference voltage, and the first range may be dependent upon a trend in measured voltage. The first range may be dependent upon a change in voltage occurring after an increase in power. Moreover, the first range may be bounded between minimum and maximum limits. [0013] The method may further involve performing the method periodically, and defining a period for performing the method periodically. Defining the period may include defining the period as a function of the power supplied to the load. The method may involve increasing the period when the power supplied to the load is relatively low and decreasing the period when the power supplied to the load is relatively high. [0014] The method may further involve adjusting the reference voltage periodically, or may involve increasing the reference voltage when a change in power drawn from the energy converter results in a change in supply voltage within a second range. The second range may be dependent upon the amount of power being supplied to the load. The second range may be relatively small when a relatively large amount of power is supplied to the load and the second range may be relatively large when a relatively small amount of power is supplied to the load. The amount by which the reference voltage is decreased may be dependent upon the amount of power supplied to the load. The amount by which the reference voltage is decreased may be relatively large when the amount of power supplied to the load is relatively low and the amount by which the reference voltage is decreased may be relatively low when the amount of power supplied to the load is relatively high. [0015] In accordance with another aspect of the invention there is provided an apparatus for controlling an energy transfer device operable to draw electrical energy from an energy converter operable to convert energy from a physical source into electrical energy and supply the electrical energy to a load. The apparatus includes a load power sensor operable to measure power supplied to the load by the energy transfer device, a voltage sensor operable to measure a supply voltage the energy converter, and a processor, in communication with the voltage sensor, the load power sensor and the energy transfer device. The processor is configured to cause the energy transfer device to change the amount of power drawn from the energy converter when the supply voltage of the energy converter meets a criterion, wherein said criterion and the change in power drawn from the energy converter is dependent upon a present amount of power being supplied to the load. [0016] The processor may be configured to decrease the power drawn from the energy converter by an amount corresponding to a change in the power supplied to the load in a time interval. Alternatively, the processor may be configured to increase the power drawn from the energy converter by an amount associated with a range of power supplied to the load. [0017] The processor may be configured to deem that the supply voltage satisfies the criterion when the supply voltage is within a first range of voltages relative to a reference voltage. The reference voltage may correspond to a maximum power point of the energy conversion device. [0018] The first range may include voltages greater than the reference voltage, it may includes voltages less than the reference voltage, or it may include voltages less than the reference voltage and voltages greater than the reference voltage. The first range may exclude a range of voltages within a limit of the reference voltage, and may be dependent upon a trend in measured voltage. The first range may further be dependent upon a change in voltage occurring after an increase in power, and may be bounded between minimum and maximum limits. [0019] The processor may be configured to periodically measure the supply voltage and change the power drawn from the energy converter accordingly, and may be further configured to define a period for measuring the supply voltage, and the period may be defined as a function of the power supplied to the load. The processor may be configured to increase the period when the power supplied to the load is relatively low and decrease the period when the power supplied to the load is relatively high. [0020] The processor may be configured to adjust the reference voltage periodically. The processor may be configured to increase the reference voltage when an increase in power drawn from the energy converter results in a change in supply voltage within a second range. The second range may be dependent upon the amount of power being drawn from the energy converter. The second range may be relatively small when relatively large amounts of power are being supplied to the load and the second range may be relatively large when relatively small amounts of power are being supplied to the load. The processor may be further configured to decrease the reference voltage by an amount dependent upon the amount of power supplied to the load. In particular, the processor may be configured to decrease the reference voltage by a relatively large amount when the power supplied to the load is relatively low and to decrease the reference voltage by a relatively small amount when the power supplied to the load is relatively high. The apparatus may include an output operable to provide a power command signal to the energy transfer device, and the processor may be configured to produce the power command signal to represent the change in power to be drawn from the energy converter. Continue reading about Method and apparatus for controlling power drawn from an energy converter... Full patent description for Method and apparatus for controlling power drawn from an energy converter Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Method and apparatus for controlling power drawn from an energy converter patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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